Breast cancer is one of the most commonly diagnosed cancers in pregnant women. Diagnosis and treatment in these women are particularly challenging as the necessary investigations and treatment modalities may potentially harm the developing foetus. Nonetheless, it appears that initial diagnostic investigations may be carried out as for non-pregnant women, but bearing in mind the risks of exposing the foetus to ionising radiation. Surgery can be carried out with little risk to the mother or foetus but radiotherapy is generally avoided. Chemotherapy can be given relatively safely when administered after the first trimester, but large volume prospective data and, in particular, data regarding the long-term implications of in utero exposure, are awaited.

Tamoxifen can be considered a classic ‘pro-drug’, requiring metabolic activation to elicit pharmacological activity. Our findings suggest that both genetic and drug-induced factors that alter CYP2D6 enzyme activity influence the clinical response to tamoxifen. Given the small differences in disease-free survival comparing tamoxifen to third-generation aromatase inhibitors, knowledge of the genetic and environmental factors that influence CYP2D6 enzyme activity may provide a robust tool to individualize the hormonal therapy to breast cancer.